Method for automatically selecting color calibrations

ABSTRACT

In a method for automatically selecting color calibrations for digital color image recording, especially for digital photography or digital video recording in any lighting conditions, at least one color calibration is produced and stored by recording a calibrated color table. The color calibration which best resembles the color cast of a color image is selected on the basis of the correlation between the color cast parameters of the color image which is being recorded and the respective color calibration. In the event that no matching color calibration is found by comparing the color cast parameters, a standard color calibration is selected.

BACKGROUND OF THE INVENTION

The invention is directed to a method for the automatic selection ofcolor calibrations for digital color image acquisition and color imagerecording, particularly for digital photography or digital videorecording under arbitrary lighting conditions.

In digital color image acquisition, it is necessary to undertake apost-processing and correction of the digital image data in the area ofthe signal processing. For that purpose, the device-dependent digitalimage data are transformed into a standardized, device-independent colorsystem, for example into the CIELAB system. Such a transformation isundertaken on the basis of a color calibration. A color calibration fora device for color image acquisition is acquired in that a calibratedcolor table having a plurality of color fields whose CIELAB color valuesare known is registered with the device. The image data of the take forthe individual color fields in the device-dependent RGB system (red,green, blue) are interpreted and allocated to the known CIELAB colorvalues of these color fields. The color calibration derives from thisallocation, i.e. which CIELAB color data belong to specific RGB colordata that the device generates when taking a picture is now known. Thetransformation of the take data of an image with the color calibrationacquired in this way simultaneously effects a color correction of theimage.

The transformation of registered RGB image data into CIELAB color dataon the basis of a selected color calibration can occur in an imageprocessing station but can also occur directly in a device for takingpictures, for example in a digital camera. Two methods are currentlyemployed.

In the first method, every lighting condition present at the acquisitionlocation is calibrated, i.e. the calibrated color table is firstphotographed under the current lighting and a color calibration matchingthis lighting is acquired therefrom and stored. The pictures aresubsequently taken and stored. The images are then either immediatelytransformed into CIELAB color data with the color calibration acquiredimmediately therebefore or later in a processing station. Very goodresults with respect to the color quality are achieved with this method.However, lighting conditions can change very fast, for example due toclouds or the like given exterior shots, so that the determination of anew color calibration is required. This method has the disadvantage ofgreatly limiting the photographer since he needs additional material(the calibrated color table), additional knowledge and additionalexposure time.

In the second method, color calibrations are defined for a few,permanently set lighting conditions, usually only daylight andartificial light, and are automatically or manually selected with animage light analysis. However, the results that are thereby achieved arenot adequate given high demands or unusual lighting. Thea advantage ofthis method is that the user can work freely without having toconstantly register color tables.

This second method is described in greater detail below. There areseveral possibilities for realizing the method, whereby there are twocorrection modules that are employed in combination or individually. Thecorrection modules are color calibration and color cast compensation.Even when the correction modules are combined, they work independentlyof one another according to the Prior Art, i.e. they are selectedindependently of one another and are then successively applied.

When an image has a color cast, then a color cast compensation isundertaken. A color cast is usually determined in that the histograms ofthe red, green and blue part of the color image data are investigated.FIG. 1 shows such histograms as an example. The densities of the colorparts are entered on the horizontal axis, i.e. the logarithmized lightintensity data. The frequencies of occurrence with which the densitiesoccur in the image are entered on the vertical axis. Corner values forthe darkest color values (image dark) and the brightest color values(image light) in the image are derived from the histograms. For example,the density at which the accumulated histogram has reached 5% is takenas image dark, and the density at which the accumulated histogram hasreached 95% is taken as image light. The corner values acquired in thisway are parameters that describe the color cast of an image. In theexample of FIG. 1, the red part in the image light has a higher densitythan the green part and the blue part. The image thus has a red cast.

The color cast compensation is generally undertaken in that each primarycolor (RGB) is modified independently of one another by a linearfunction that is implemented on intensity-linear data, i.e. data thatcorrespond to the sensor signals in the acquisition unit and areproportional to the intensity of the incident light. The colors arethereby provided with an offset and intensified such that the cornervalues given image light and/or image dark coincide after application ofthe function, i.e. image light and/or image dark subsequently lie on thegray axis. The linear functions for the color cast compensation can berealized as look-up tables that allocate modified color part data to theas yet unmodified color part data.

The color transformation on the basis of a color calibration can beimplemented in addition to the color cast compensation or alone, i.e.without a preceding or following color cast compensation. The colortransformation is a multi-dimensional allocation of color data, forexample of CIELAB color data, to the RGB image data. Dependenciesbetween the primary colors are thereby taken into consideration. Thecolor calibration is generally designed as a multi-dimensionalallocation table that is also referred to as a calibration fill. Thecolor calibration, however, can also be defined by mathematicalfunctions, for example by matrix multiplication. A color calibration canimplicitly contain a color cast compensation that is worked into themulti-dimensional allocation table. No separate color cast compensationneed be undertaken then. However, the color calibration can also beproduced such that it assumes data free of color cast at the input andoutput of the transformation. In this case, a separate color castcompensation is implemented.

According to the Prior Art, the color calibration for digital imageacquisition systems is based to a great extent on traditionalphotography. Daylight films are mainly employed in traditionalphotography. They are designed for a lighting corresponding to a colortemperature of 5000-6000 K. This color temperature is delivered by allphotoflash systems and under good daylight conditions. Artificial lightfilms that are designed for approximately 3000 K and are used underhalogen lighting are offered as the only widespread alternative todaylight films. Corresponding to the photographic standard, the digitalimage acquisition systems are calibrated for a color temperature of5000-6000 K, i.e. the acquired image data are transformed with a colorcalibration matching this lighting. Slight deviations of the lightingare then potentially additionally compensated with a color castcompensation. Even despite the implementation of a color castcompensation, greater deviations lead

SUMMARY OF THE INVENTION

It is an object of the invention to acquire digital image data givenarbitrary lighting conditions without burdening the user with thecalibration operations needed due to a change in the lighting, wherebythe further-processing of the images can automatically occur and a highcolor quality is achieved.

This object is achieved by a method for the automatic selection of colorcalibrations for digital color image acquisition, particularly fordigital photography or digital video recording, under arbitrary lightingconditions, whereby a color image is to be corrected. Features of theinvention are characterized in that at least one color calibration isgenerated in that an analysis of the color cast of the data of a take ofa calibrated color table is implemented, a compensation of the colorcast is undertaken and parameters for the color cast compensation arestored, at least one calibration fill is calculated with a calibrationsoftware from the color cast-compensated data of the take of thecalibrated color table and is stored, and an entry about the analyzedcolor cast is respectively undertaken in the calibration fill or fills.A processing of the acquisition data of a current image occurs in thatthe color cast of the current image is analyzed and compared to thecolor casts of the stored color calibrations by means of a correlationand a selection from the stored color calibrations is implemented,whereby that color calibration whose stored color cast has the bestapproximation to the color cast of the current image is selected, andthe selected color calibration is provided for generating adevice-independent color transformation. When no color calibrationhaving an adequately good correlation of the color cast stored with thecolor calibration is found in the comparison of the color cast of thecurrent image to the color casts of the stored color calibrations, theuser is informed thereof, so that the user can select a suitable colorcalibration, or a standard calibration is automatically selected fromthe stored color calibrations and provided for generating adevice-independent color transformation of the image data of the currentimage.

The color cast of the current image is then compared to the color castof the selected color calibration. The color cast parameters areselected for the transformation given good correlation, and the colorcast parameters of the selected color calibration are selected for thetransformation given poor correlation. Subsequently, thedevice-independent color transformation is implemented with the selectedcolor cast parameters and with the selected color calibration.

The invention is described in greater detail below with reference to theexample of a digital camera as a color acquisition device. The sameprinciple, however, can also be applied to digital video recording. Thenew method functions given arbitrary types of light for theillumination, which can deviate greatly from the ideal black body. Theprevious, color-correct working range of a system (approximately5000-6000 K) is expanded by arbitrary light types. The description ofthe invention also occurs with reference to FIGS. 1 through 5. Showntherein are:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows histograms for the color components red, green and blue ofan acquired image;

FIG. 2 is a flowchart for generating a color calibration;

FIG. 3 is a flowchart for selecting a color calibration;

FIG. 4 shows an example of a good correlation; and

FIG. 5 is an example of a poorer correlation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to the invention, a lighting illuminant is defined by a colorcalibration. The color calibrations are generated in that a calibratedcolor table is photographed under lighting with the respectiveilluminant, and the color calibration of the ICC specification isacquired from the data of the take. The exact description of theproduction of the color calibrations shall not be embarked upon here.The product description “ScanOpen” ICC Application of HeidelbergerDruckmaschinen AG, D-34107 Kiel is referenced with respect thereto, thisbeing obtainable under the identification number 05226870, version ofMay 1997. The publication “ICC Profile Format Specification”, Version3.4, Aug. 15, 1997, International Color Consortium (ICC, web site:http://www.color.org) is also referenced in this context. A calibratedcolor table has been standardized by both ANSI as well as ISO under thedesignation IT8. The color calibration images both the light conditionsin the exposure as well as the color behavior of the sensor and of thesignal processing in the acquisition device.

The color calibrations are produced for various ruminants, for example

artificial light/tungsten (approximately 3000 K)

standard light (approximately 5000 K)

sunlight (approximately 6000 K)

shadow (approximately 8000 K).

During the production of the calibration, the color cast is analyzed foreach exposure of the calibrated color table, and the corresponding colorcast parameters for image light and image dark are identified. The colorcast is essentially produced by the color spectrum of the light of theexposure. A compensation of the color cast is then undertaken, and acalibration fill is calculated with a calibration software using thecolor cast-compensated data of the take of the calibrated color tableand is stored. The identified color cast parameters are stored togetherwith the calibration fill. FIG. 2 shows these steps again as a structurediagram according to Nassi Schneidermann. The production of such colorcalibrations can be repeated or expanded arbitrarily often. Theproduction can be implemented both by the manufacturer as well as by theuser.

The inventive method for the selection of a suitable color calibrationfor a currently acquired image is described below. The method steps aresummarized in FIG. 3 in a structure diagram.

In step S1, a color cast analysis of the current image is implementedfor image light and image dark, for example by evaluating the histogramsfor the red, green and blue color components. Alternatively, color castinformation offered by the camera electronics of color cast informationthat are acquired from picture elements that the user specificallyselects for this purpose can be employed.

In step S2, a search is made for the color calibration that fits bestwith respect to the color cast compensation. For that purpose, thestored color cast parameters of all existing color calibrations arecompared to the color cast parameters of the current image on the basisof a correlation function. FIG. 4 shows an example for the formation ofthe correlation function. The color cast corner values for image lightof the current image and the corresponding corner values of the colorcalibration being investigated at the moment are entered in a diagramand respectively connected to one another by the straight lines (1)through (6). The slopes of the straight lines are then determined. Theyare referenced A1 through A6 here. The function

K=abs(A1-A4)+abs(A2-A5)+abs(A3-A6)

is then formed as correlation value K, i.e. the sum of the absoluteslope differences of the straight lines (1) and (4), (2) and (5) as wellas (3) and (6) that correspond to one another. A good correlation isestablished when the correlation value is as small as possible. Thecorrelation value is then compared to a first threshold T1 in order todecide whether the correlation is good enough. When this is not thecase, the next existing color calibration is investigated. When thecorrelation is good enough, the correlation value is compared to that ofthe best color calibration hitherto found. When the correlation of thecurrently investigated color calibration is better, it is noted for thelater selection.

In step S3, a determination is first made as to whether a suitable colorcalibration was found in step S2. When this is not the case, the user isinformed thereof, so that the user can select a suitable colorcalibration on the basis of his experience, or the system selects astandard color calibration provided for this case. When a suitable colorcalibration was found in step S2, this is presented to the user forselection or it is automatically selected right away.

In step S4, the color cast parameters of the current image and thepreviously selected color calibration are again compared separately forimage light and image dark in order to decide which color castparameters are used for the color cast compensation of the currentimage. For that purpose, the correlation value K for image light isfirst compared to a second threshold T2. When the correlation is goodenough, the color cast of the current image in the image light area iscorrected with the color cast parameters of the current image.Otherwise, the correction is implemented with the color cast parametersof the selected color calibration. Subsequently, the same comparison andthe correction of the color cast with the better-suited parameters isimplemented for the image dark area.

Finally, the selected color calibration is applied to the colorcast-corrected image data of the current image in order to transformthem into a device-independent color system. A color-correctpresentation of the image is thus achieved.

The inventive method can be expanded and improved in that additionaltake parameters are also utilized for the comparison in addition to thecomparison of the color cast parameters for selecting the best-suitedcolor calibration. For example, information such as diaphragm, exposuretime, whether the shot was made with flash and with which flash settingscan be stored together with the generated color calibration in thephotograph of the calibrated color table and can be compared later tothe corresponding information of the current image acquisition. Inaddition, the time of day of the photograph (morning, during the day,evening, night) can be retained as further information that sayssomething about the light conditions when the picture was taken. Forexample, all parameters can then be operated according to the principlesof fuzzy logic in order to arrive at an even more reliable decisionabout the selection of the correct color calibration.

Although various minor changes and modifications might be proposed bythose skilled in the art, it will be understood that my wish is toinclude within the claims of the patent warranted hereon all suchchanges and modifications as reasonably come within my contribution tothe art.

I claim:
 1. A method for automatic selection of color calibrations fordigital color image acquisition under arbitrary lighting conditions andfor correction of take data of a color image with the color calibrationsbased on a color cast compensation and a color transformation, wherebyit is not only neutral colors that are utilized for the color castcompensation, comprising the steps of: producing at least one colorcalibration by acquiring a calibrated color table under defined lightingconditions; implementing an analysis of the color cast of the take dataof the color table and determining parameters that describe the colorcast of the take data of the color table; undertaking a compensation ofthe color cast of the take data of the color table; calculating at leastone color calibration from the color cast-corrected take data of thecolor table and storing it; storing the determined parameters of thecolor cast of the take data of the color table together with the colorcalibration; determining color cast parameters of the take data of thecolor image and comparing them to the stored color cast parameters ofthe color calibrations; and undertaking a selection of a colorcalibration on the basis of the comparison.
 2. The method according toclaim 1 wherein the comparison of the color cast parameters isimplemented with a correlation.
 3. The method according to claim 1wherein the color calibration whose stored color cast parameters arecorrelated best with the color cast parameters of the color image isselected.
 4. The method according to claim 1 wherein a standard colorcalibration is selected when an adequately good correlation of the colorcast parameters is not found in the comparison of the color castparameters of the color calibrations and of the color image.
 5. Themethod according to claim 1 wherein the user is informed when anadequately good correlation of the color cast parameters is not found inthe comparison of the color cast parameters of the color calibrationsand of the color image, so that the user himself can select a suitablecolor calibration.
 6. The method according to claim 1 wherein the colorcast parameters of the color image are compared to the color castparameters of the selected color calibration, and the color castparameters of the color image are employed for the color castcompensation of the color image given good correlation and the colorcast parameters of the selected color calibration are employed for thecolor cast compensation of the color image given a poor correlation. 7.The method according to claim 1 wherein the comparison and the selectionof the color cast parameters employed for the color cast compensation isundertaken separately for the image light area and for the image darkarea of the color image.
 8. The method according to claim 1 wherein acolor transformation of the take data of the color image is implementedwith the selected color calibration.
 9. The method according to claim 1wherein at least one additional photographic parameter selected from thegroup diaphragm, exposure time, distance, flash settings, and time ofday of the exposure is involved in the comparison of the colorcalibrations to the color image.
 10. The method of claim 1 wherein themethod is employed for at least one of digital photography and digitalvideo recording.
 11. A method for the automatic selection of colorcalibrations for digital color image acquisition under arbitrarylighting conditions, and for correction of take data of a color imagewith the color calibrations on the basis of a color cast compensation,comprising the steps of: producing at least one color calibration byacquiring a calibrated color table under defined lighting conditions;implementing an analysis of the color cast of the take data of the colortable and determining parameters that describe the color cast of thetake data of the color table; undertaking a compensation of the colorcast of the take data of the color table; calculating at least one colorcalibration from the color cast-corrected take data of the color tableand storing it; storing the determined parameters of the color cast ofthe take data of the color table together with the color calibration;determining color cast parameters of the take data of the color imageand comparing them to the stored color cast parameters of the colorcalibrations; and undertaking a selection of a color calibration on thebasis of the comparison.